Remove empty constructor.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188232 91177308-0d34-0410-b5e6-96231b3b80d8
Reflow comment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188233 91177308-0d34-0410-b5e6-96231b3b80d8
Add comment and source to testcase.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188234 91177308-0d34-0410-b5e6-96231b3b80d8
Fix an oversight in isPotentiallyReachable where we wouldn't do any CFG-walking
to find loops if the From and To instructions were in the same block.

Refactor the code a little now that we need to fill to start the CFG-walking
algorithm with more than one starting basic block sometimes.

Special thanks to Andrew Trick for catching an error in my understanding of
natural loops in code review.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188236 91177308-0d34-0410-b5e6-96231b3b80d8